Clinical Biochemistry - Students

Creatinine is not re-utilised in the body's metabolism and hence is solely a waste product which is removed in the kidneys.

Plasma creatinine is thus determined by the rate of creatinine production in muscles and by the rate of elimination in the kidneys. The amount of creatinine in the plasma will thus be influenced by the muscle mass of the patient and kidney function. Hence the reference range for males is higher than for females.

Creatinine levels in plasma will go up if kidney function is compromised and hence the use of creatinine to give an indication of renal function. Note that the filtering action of the kidneys, the glomerular filtration rate (GFR), can be reduced by half before plasma creatinine increases.

GFR is widely accepted as the best measure of kidney function. To determine it accurately requires injecting a substance into a patient and looking at its clearance into urine. This is not practical in the everyday clinical situation and so the measurement of creatinine is performed to estimate the GFR.